The latest predictions are published by the population division of the UN's Department of Economic and Social Affairs.

Improving longevity

The report suggests that if fertility levels stabilise at around two children for every woman the population increase will be more manageable, reaching just over nine billion people in three centuries' time.

The continuing rise in this case can be explained by improving longevity.

The number of people over 60 would rise from 10% to 38%, and those over 80 from 1% to 17%.

But even small variations in fertility levels either side of this could radically alter these figures, the report said.

An average fertility of 1.85 children per woman would result in a population of just 2.3 billion, whereas an average of 2.35 would yield 36.4 billion.

The report says that whatever the overall increase, the world's population is likely to be significantly older in 300 years.

Almost a quarter of this population will live in Africa while India, China and the United States will continue to be the most heavily populated countries of the world.